The most commonly used explosive in mining is a mix of ammonium nitrate and fuel oil (ANFO). The fuel oil typically used is no. 2 fuel oil but diesel fuel, kerosene and vegetable oils have been used. ANFO is a high energy explosive which produces a large shock wave component as part of the explosive energy which is released.
The shock component is somewhat inefficient in the blasting of rock and also results in a higher proportion of fine dust and debris being raised from the blast which is clearly undesirable from a safety and environmental point of view. When mining for valuable materials such as gold or diamonds it is also desirable to reduce the shock wave component as this can result in damage to or loss of some of the material. The heave component of the explosive energy effectively does most of the necessary work in expanding cracks in the rock and lifting the burden and so it is desirable to lessen the explosive energy which goes into the shock wave and maximise that in the heave component.
There have been attempts in the past to modify explosive compositions to produce a low shock energy explosive (LSEE) with limited success. Dilution of the explosive mixture with materials such as sawdust or rice husks have helped to reduce the shock energy but the dilution effect means that more composition is needed to achieve the same effect and so larger or greater numbers of boreholes must be drilled in which to locate the explosive. These low density explosives also result in a lowering of the blast efficiency.
U.S. Pat. No. 5,505,800 presents a partial solution to this problem by disclosing the use of a combination of an oxidising agent, such as ammonium nitrate, and a particulate solid fuel which is selected from materials such as rubber, gilsonite, unexpanded polystyrene and the like. These solid fuels are used to replace preferably all of the liquid fuel oil and, since they are slower burning than the liquid fuel oils, result in an increase in the time during which the explosive pressure builds up which causes a significant reduction in the shock energy produced. Rubber is the preferred fuel in combination with ammonium nitrate leading to this formulation being referred to as ANRUB.
This is still not an ideal situation as uneven explosion characteristics are obtained along with unpredictable overall blasting efficiency. The use of different solid fuel particles for different purposes results in a blast result that can be unpredictable and highly variable.